The present invention relates to an improved process for the preparation of tetramethoxybutene by reacting 2,5-dimethoxydihydrofuran with methanol in the presence of solid catalysts having acidic centers.
Tetramethoxybutene is an important intermediate for preparing C10-dialdehyde of the formula 
which in turn is a key building block for the synthesis of carotenoids such as xcex2-carotene, astaxanthin and lycopene.
According to the process as described by C. M. Cox, D. A. Whiting, J. Chem. Soc. Perkin Trans. 1 1991, 1907-1911, V. M. Likhosherstov, Russ. J. Org. Chem. 1983, 19, 1176-1178, and S. M. Makin, N. I. Telefina, Z h. Obshch. Khim. 1962, 32, 1104-1109, and in U.S. Pat. No. 2,768,976 and DE 956 946, tetramethoxybutene is prepared from furan and bromine in methanol. Dimethoxydihydrofuran is formed in situ after 1,4-addition of bromine to the furan and subsequent nucleophilic substitution of the bromine by methanol. Owing to the formation of Br during the bromine substitution, the dimethoxydihydrofuran immediately reacts further to give tetramethoxybutene.
This process has serious disadvantages: the synthesis has to be conducted at temperatures of from xe2x88x9230 to xe2x88x9250xc2x0 C., which is difficult to realize technically: the use of bromine requires a high expenditure on safety. Furthermore, bromine is an expensive reagent and highly corrosive, and it is necessary to equip the production plant with expensive specialty materials. In addition, the hydrogen bromide which is formed in equimolar amounts has to be neutralized during work up, producing large amounts of waste salts. It is in principle possible to conduct this synthesis using less expensive chlorine, but the disadvantages described for bromine remain, and the reaction is also significantly slower.
Since dimethoxydihydrofuran can advantageously be obtained by oxidation of furan in methanol, it has also been described to convert dimethoxydihydrofuran into tetramethoxybutene in the presence of a strong dissolved acid, e.g. p-toluenesulfonic acid or hydrogen chloride. Since this reaction which corresponds to the following reaction equation 
is a particular type of a transacetalization with formation of one mole of water, the water which is released must be removed from the equilibrium. According to the process of EP-A 0 581 097, this problem is solved by adding trimethyl orthoformiate, which is relatively expensive. Furthermore, it is not easy to transfer this process to the continuous scale. Here it is also necessary to neutralize the dissolved catalyst by the addition of a base to be able to stop the reaction at its optimum. Finally, N. Clauson-Kaas, J. T. Nielsen, E. Boss, Acta Chem. Scand. 1955, 9, 111-115, describe the use of aprotic Lewis acids, e.g. boron trifluoride, but the tetramethoxybutene yield was only 9% of theory.
Another problem of the prior art processes is the formation of considerable amounts of the byproduct pentamethoxybutane, reducing the tetramethoxybutene yield.
It is an object of the present invention to provide a process which allows the technically simple preparation of tetramethoxybutene, in particular in a continuous manner and in good yield, while reducing the formation of pentamethoxybutane.
We have found that this object is achieved by carrying out the reaction in the presence of solid catalysts having acidic centers. This results not only in high selectivities, in particular at partial conversion, but also in lower amounts of pentamethoxybutane byproduct. Unconverted dimethoxybutene can be returned to the reaction after distillation of the reaction products with removal of the water which has formed.
More specifically, the novel process relates to the preparation of 1,1,4,4-tetramethoxy-2-butene by reacting 2,5-dimethoxydihydrofuran with methanol in the presence of acids, which comprises carrying out the reaction in the presence of solid catalysts having acidic centers.
Solid catalysts having acidic centers are in particular acidic organic ion exchangers or inorganic oxidic catalysts which have acidic centers and are selected from the group consisting of zeolites in the H form, acidic mixed oxides and sheet silicates having acidic centers.
The catalysts to be used according to the invention essentially belong to four groups consisting of
a) acidic organic ion exchangers which are preferred,
b) zeolites in the H form,
c) acidic mixed oxides, and
d) sheet silicates having acidic centers.
a) Acidic Organic Ion Exchangers
Acidic organic ion exchangers are conventional, partially crosslinked chain polymers which are derived from styrene/divinylbenzene, in particular, and which contain preferably sulfonic acid groups, e.g. of the formula 
Ion exchangers of this type are commercially available, e.g. from DOW Chemical or BAYER Aktiengesellschaft.
Examples are Dowex(copyright) 50WX, Serdolit Red(copyright), Amberlyst(copyright) 15, Lewatit(copyright) K2431, Navion(copyright) HL, Amberlite(copyright) IR120, Duolite(copyright) C20, Lewatit(copyright) S100 and Lewatit(copyright) K2641.
b) Zeolites in the H Form
Preference is given to the acidic H form of 12-ring zeolites of the structure type BETA, Y, EMT and mordenite and 10-ring zeolites of the pentasil type. As well as the elements aluminum and silicon, zeolites can also contain boron, gallium, iron or titanium in their framework. Furthermore, they can also be partially exchanged with the elements of group IB, IIB, IIIB, IIIA or VIIIB and the lanthanide elements.
Zeolites to be used as catalysts include, for example, zeolites in the acidic H form of the structure type MFI, MEL, BOG, BEA, EMT; MOR, FAU, MTW, LTL, NES, CON or MCM-22 according to the structure classification given in W. M. Meier, D. H. Olson, C h. Baerlocher, Atlas of zeolite Structure Types, Elsevier, 4th ed., 1996.
Particular examples are the zeolites ZBM-20, Fe-H-ZSM5, Sn-beta zeolite, beta zeolite, Zr-beta zeolite, H-beta zeolite, H-mordenite, USY, Ce-V zeolite, H-Y zeolite, Ti/B-beta zeolite, B-beta zeolite or ZB-10.
c) Acidic Mixed Oxides
The acidic mixed oxides to be used according to the invention are in particular superacidic mixed oxides which have been described repeatedly in the literature. Reference may be made, for example, to R. J. Gillespie, Acc. Chem. Res. 1 (1968) 202 and R. J. Gillespie and T. E. Peel, Adv. Phys. Org. Chem. 9 (972) 1.
Specific superacidic metal oxides which can be used in the reaction of the invention are disclosed by Kazushi Arata in Applied Catalysis A: General 146 (1996) 3-32 for the reaction of butene and pentanes.
Exemplary sections of this reference which pertain to the preparation of the superacidic metal oxides are reproduced susequent to the examples. Additional information provided by Arata on the properties of the superacidic metal oxides, as well as sources referenced by Arata have been omitted.
Suitable sulfatized or phosphatized metal oxides (i) are in particular phosphatized or sulfatized zirconium oxide or titanium oxide which may include further elements such as iron, cobalt or manganese.
Preferred sulfatized or phosphatized catalysts are:
Preference is given to superacidic mixed metal oxides of groups (i) and (ii) which contain zirconium, titanium, iron, tin or Cr(III) on the one hand and tungsten or molybdenum on the other.
Specific examples are TiO2WO3, Fe2O3WO3, ZrO2MoO3, ZrO2WO3, Cr2O3WO3, WO3TiO2, TiO2WO3 or SnO2WO3SiO2, the molar ratio of the oxides of group (i) to group (ii) usually being from 70:30 to 90:10.
In accordance with one embodiment of the process, the catalyst comprises, as an essential constituent, mixed oxides having acidic centers, which oxides consist of a combination of
(i) oxides of titanium, zirconium, hafnium, tin, iron or Cr(III), and
(ii)oxides of vanadium, chronmium(VI), molybdenum, tungsten or scandium,
or the mixed oxides are sulfatized or phosphatized oxides of group (i), and the mixed oxides have been calcined at from 459xc2x0 C. to 800xc2x0 C. after combining.
d) Sheet Silicates Having Acidic Centers
For the purposes of the invention, sheet silicates having acidic centers are those having Lewis and/or Brxc3x6nsted centers. Therefore, they may be sheet silicates to which said Lewis acids have been applied or which have been treated with acids such as sulfuric acid. However, preference is given to sheet silicates which have negative layer charges neutralized by protons. In this case, the acidic centers of the sheet silicates are essentially Brxc3x6nsted centers formed in the sheet silicates having excess negative charges by exchange of the metal ions for protons.
The sheet silicates to be used according to the invention are especially aluminum silicates; they belong to the clay minerals and are composed of SiO2 tetrahedron and Al2O3 octahedron layers, part of the silicon in the tetrahedron layer being replaced by trivalent cations, preferably aluminum, and/or part of the aluminum in the octahedron layer being replaced by bivalent cations, preferably magnesium, so that negative layer charges result.
Sheet silicates having negative charges occur naturally as montmorillonites, vermiculites or hectorites or can be prepared synthetically.
A more detailed description is given in Z. M. Thomas and W. Z. Thomas, Principles and Practice of Heterogeneous-Catalysis, 1997, Vetc. ISBN 3-527-29239-8, p. 347 ff.
However, preference is given to naturally occurring montmorillonite which is converted into its H form by treatment with acids.
An example is montmorillonite of the formula
Na0,33{(Al1,67Mg0,33)(OH)2[Si4O10]}
having layer charges from about 0.6 to 0.2 per formula unit.
To partially or completely neutralize the negative layer charges with protons, the exchangeable cations, usually alkali metal or alkaline earth metal ions, in the naturally occurring or synthetic sheet silicates are exchanged for protons. This is done in a conventional manner, e.g. by treatment with sulfuric acid or hydrochloric acid.
Since the sheet silicates containing protons instead of alkali metal or alkaline earth metal ions are thermally less stable, it is also possible to use pillared clays in which the layers are supported against one another. The preparation of such pillared clays is described in detail in Figuras, Catal. Rev. Sci. Eng. 30 (1988) 457 or Jones, Catal. Today (2 (1988) 357, which are incorporated here in the reference.
Specific examples of sheet silicates having negative layer charges which are neutralized by protons are: montmorillonite, vermiculite and hectorite.
The catalysts to be used according to the invention are employed in pulverulent or preferably particulate form, for example in the form of granules, extrudates or spheres.
The heterogeneous catalyst maintains its activity over a prolonged period of time. The inorganic catalysts can then be reactivated, for example by burning off in air at above 450xc2x0 C. For these reasons the novel process is economically and environmentally particularly advantageous.
The reaction is either carried out batchwise, for example as a suspension process, or preferably over a fixed-bed catalyst, in a continuous flow reactor.
In the batchwise reaction, the catalyst is typically used in amounts of from 1 to 50, preferably from 5 to 30, % by weight, based on dimethoxydihydrofuran. Methanol is generally used in excess over the stoichiometrically required amount, for example in 2 to 40 times the molar amount, preferably in a molar excess of from 2 to 20, in particular from 1.5 to 10.
However, preference is given to the continuous process for which only a slight excess of methanol is required without substantially reducing the yield. It is therefore possible to carry out the reaction at a molar ratio of dimethoxydihydrofuran to methanol of 1:2-1:4, preferably 1:2.4-1:4.
The reaction according to the invention usually takes place at from xe2x88x9210 to 100xc2x0 C., preferably from 0 to 40xc2x0 C., in particular from 15 to 30xc2x0 C., and at residence times of from 1 to 30 min, preferably from 10 to 60 min.
In the batchwise embodiment of the invention, dimethoxydihydrofuran is stirred together with an excess of methanol, e.g. in combination with an acidic ion exchanger, at e.g. 0-25xc2x0 C. for several hours, the catalyst is filtered off and the reaction mixture is worked up by distillation. Unconverted dimethoxydihydrofuran can be reused.
However, it is advantageous to carry out the reaction in a continuous manner by passing dimethoxydihydrofuran together with methanol over a fixed-bed acidic catalyst. The reaction is advantageously carried out only to a partial conversion, e.g. less than 80% of theory. The mixture leaving the reactor is worked up by distillation and the unconverted dimethoxydihydrofuran and the dehydrated methanol are returned to the reaction.
Preferred reactors are tubular reactors in which the acidic catalyst is arranged in one or more beds.